TERT transcriptional deregulation in thyroid cancer progression I trained in thyroid cancer genetics in my graduate studies, during which time I identified germline promoter variants conferring thyroid cancer susceptibility. In my postdoctoral years I led a project that revealed the key somatic genomic and transcriptomic hallmarks of poorly differentiated (PDTC) and anaplastic (ATC) thyroid cancers. We reported that mutations in the promoter of TERT (telomerase reverse transcriptase) gene are the most prevalent genetic alterations in often fatal PDTCs and ATCs, and constitute good biomarkers of disease severity. Although it is widely accepted that TERT promoter mutations activate gene transcription, the specific mechanisms and biological effects of these alterations remain largely unknown. This prompted me to study the role of TERT transcriptional deregulation in thyroid cancer progression, which is the subject of my K22 proposal. I believe this project has great potential and will facilitate my setting up my own lab. To this end, I have put together an advisory committee and collaborators who will share their expertise and resources in the fields of transcriptional regulation, chromatin biology and genetically engineered mouse models of cancer, as well as career advice, while I seek to secure a tenure-track position. This proposal outlines a comprehensive approach to characterizing the role of TERT promoter mutations in telomerase transcriptional deregulation and thyroid cancer progression. The first aim of this project is to identify the transcriptional complex that differentially binds TERT mutant vs. wildtype promoter, and to this end I propose a dual regulation model. First, we describe experiments to define the role of specific ETS family members in TERT mutant promoter reactivation, which we believe are distinct from those reported in other disease contexts. We will also evaluate the effect of MAPK constitutive signaling, a hallmark of thyroid tumors, in ETS-mediated TERT expression. Second, we aim to unveil the influence of the CTCF genome insulator on TERT transcriptional repression through long-range enhancer- promoter interactions. We have preliminary data suggesting that ETS and CTCF factors compete for TERT promoter binding in mutant and wildtype conditions, respectively, inducing opposite effects on gene transcription. Our second aim is to characterize the first Tert mutant promoter mouse model, which we have already generated via CRISPR knock-in of the equivalent mouse locus. Our preliminary data suggest that Tert promotes thyroid cancer progression in combination with a thyroid-specific oncogenic BrafV600E allele. We will also assess which signaling pathways are involved in Tert-induced thyroid tumors, and whether they create unique vulnerabilities that can be exploited therapeutically. Overall, my goal is to establish myself as an independent investigator in the field of thyroid cancer biology and transcriptional regulation, and the awarding of the K22 grant will greatly facilitate this transition.